High oxidation state alkylidene and alkylidyne metal complexes have been known for about 40 years. Alkene and alkyne metathesis via these metal complexes has been studied extensively. Alkylidyne metal complexes have been studied to a lesser extent than their alkylidene analogues, but they are of particular interest for their potential to promote nitrile-alkyne cross metathesis (NACM), which constitutes a potentially valuable tool to prepare novel alkynes from readily accessible nitriles.
Metal-alkylidynes contain a metal-carbon triple bond. Metal-alkylidynes, having a metal in its highest oxidation state, are known as Schrock-type metal-alkylidynes, and have been widely investigated. In high-oxidation state metal-alkylidynes, the alkylidyne carbon is a 6-electron donor that provides π-donation to the metal center. In spite of extensive π-donation, most high-oxidation state metal-alkylidynes are electron deficient and must be stabilized by additional ligands.
Schrock-type metal-alkylidynes are generally formed by the deprotonation of an α-CH, where a base deprotonates the α-carbon to form an alkylidyne from the alkylidene, or by an α-elimination reaction, in which bulky alkyl groups promote deprotonation of the α-CH to release steric crowding during formation of metal-alkylidynes. In rare cases, these complexes have been formed by a metathesis reaction between an alkyne and a metal-metal triple bond, or by a reductive recycle series of reactions, where a gem-dichloride reacts with a metal complex, to form a mixture of a metal chloride complex and a metal alkylidene, followed by reduction of the metal chloride complex back to the original metal complex.
A catalytic NACM was reported by Geyer et al., J. Am. Chem. Soc. 2007, 129, 3800-1, where a tungsten-nitride of the form (RO)3W≡N was found to reversibly convert to the corresponding metal-alkylidyne upon treatment with an alkyne. Unfortunately, rates of reaction were very slow and a very limited substrate scope was observed. Using a novel titanium alkylidene-alkyl complex (PNP)Ti═CHtBu(CH2tBu), where PNP is a phosphorous-nitrogen-phosphorous tridentate pincer-type ligand, and bulky nitriles, NACM was achieved. However, the catalyst required an external electrophile to liberate the alkyne, as reported by Bailey et al., J. Am. Chem. Soc. 2007, 129, 2234-5.
Polyacetylenes are organic polymers that can display electrical conductivity, paramagnetic susceptibility, optical nonlinearity, photoconductivity, gas permeability, liquid-crystallinity, and chain-helicity. Polymerization of acetylenes employs a transition metal catalyst, generally with a cocatalyst. High molecular weight polyacetylenes (>106 g/mol) have been produced from catalysts, such as M(CO)6—CCl4-hv (M=Mo, W), where the active species has been determined to be a metal-alkylidene, with polymerization involving a metathesis pathway. The metal-alkylidyne, (R3CO)3W≡CC(CH3)3, has been shown to promote alkyne metathesis and alkyne polymerization, as reported by Mortreux et al., J. Mol. Catal. A: Chem. 1995, 96, 95-105, where the product composition varied with the substitution. Polymerization was shown to be the exclusive path only with phenyl or trimethylsilyl monosubstituted acetylenes. The metal-alkylidyne, H5C6C≡W(CO)4Br, promotes a slow alkyne polymerization without alkyne metathesis as reported by Katz et al., J. Am. Chem. Soc. 1984, 106, 2659-68, but only low monomer conversion is observed after periods of days.
One might expect that OCO pincer ligand supported metal-alkylidynes should be well suited as metathesis or polymerization catalysts for alkynes, as: the trianionic nature of the OCO pincer ligand allows access to a +6 oxidation state required for a metal-alkylidyne; the rigid planarity of the OCO pincer ligand imposes geometric restraints around the metal center, which might permit an increase in reactivity; and the strong M—C bond should distort the metal-alkylidyne out of the plane of the ligand, which might further increase the reactivity of the resulting complex.